Generally, gear mechanisms, such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism as a mechanism to convert a rotary motion of an electric motor to an axial linear motion in an electric linear actuator, are used in various kinds of driving sections. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased and simultaneously size of electric motor and power consumption are also increased. Thus, the ball screw mechanisms have been widely used as more efficient actuators.
FIG. 7 illustrates an electric actuator utilizing a ball screw mechanism. The electric actuator 51 includes a housing 52 with a first housing 52a and a second housing 52b. An electric motor 53 is mounted on the housing 52. A speed reduction mechanism 57 transmits the rotational power of the electric motor 53 to a ball screw mechanism 58, via a motor shaft 53a. The ball screw mechanism 58 converts the rotational motion of the electric motor 53 into the axial linear motion of a driving shaft 59 via the speed reduction mechanism 57. The ball screw mechanism 58 has a nut 61 formed with a helical screw groove 61a on its inner circumference. The nut is rotationally and axially immovably supported via supporting bearings 66 mounted on the housing 52. A screw shaft 60 is axially integrated with the driving shaft 59. The screw shaft 60 includes a helical screw groove 60a on its outer circumference corresponding to the helical screw groove 61a of the nut 61. The screw shaft 60 is inserted into the nut 61 via a plurality of balls 62 and is axially movably but non-rotationally supported.
The electric motor 53 is mounted on the first housing 52a. A bore 63a and a blind bore 63b are formed, respectively, in the first and second housings 52a, 52b to contain the screw shaft 60. The speed reduction mechanism 57 has an input gear 54, secured on the motor shaft 53a, an intermediate gear 55 and an output gear 56, secured on the nut 61, and mating with the intermediate gear 55.
A gear shaft 64 is supported on the first and second housings 52a, 52b. Bushes 65, of synthetic resin, are interposed in either one or both of the spaces between the gear shaft 64 and intermediate gear 55 or between the first and second housings 52a, 52b and the gear shaft 64. Thus, the intermediate gear 55 can be rotationally supported relative to the housing 52. Accordingly, it is possible to provide an electric actuator 51 that can interrupt or reduce the transmission of vibration caused by play between the intermediate gear 55 and the gear shaft 64 as well as by play of gear shaft 64 itself. (See, JP2013-148108 A)
In the prior art electric actuator 51, the rotational power of the electric motor 53 is transmitted to the nut 61, of the ball screw mechanism 58, via the speed reduction mechanism 57, including the input gear 54, the intermediate gear 55 and the output gear 56. The nut 61 is rotationally supported by a pair of the supporting bearings 66 with deep groove ball bearings. The output gear 56 is arranged between the two supporting bearings 66 and secured on the nut 61, via a key. The output gear 56 contacts an inner ring 67 of one of the supporting bearings 66.
The inner rings 67 of the bearings 66 are secured on the outer circumference of the nut 61 and thus rotate together with the nut 61. On the other hand, the outer rings 68 of the bearings 66 cannot rotate since they are securely fit in the housing 52. Accordingly, smooth rotation of the output gear 56 would be impaired if the side surface of the output gear 56 contacts the end face of the outer ring 68 of the bearing 66. Thus, the output gear 56 is formed so that its axial thickness is smaller than its boss 56a that contacts the inner ring 67 of the bearing 66. This prevents contact of the output gear 56 against the outer ring 68 of the bearing 66.
Accordingly, the output gear 56 is formed so that its axial thickness is small except for its boss 56a, as shown in FIG. 8(b). In the output gear 56, with such a configuration, only the boss 56a contacts a supporting surface 69. The tooth tip portion 56b does not contact the supporting surface 69 when the output gear 56 is placed on the supporting surface 69 during manufacturing of the output gear 56. Accordingly, it is believed that a bow is created in the tooth tip portion 56b, as shown by an arrow in FIG. 9. This would impair the manufacturing accuracy of the output gear 56.
Especially in the case of a gear made of sintered alloy, such as the prior art output gear 56 used in the electric actuator 51, the configuration of the output gear 56 would deform under its own weight due to an insufficiency of the binding degree of powder for a time until desired strength has been obtained after completion of the sintering treatment in the manufacturing processes. In addition, corners of teeth of the sintered output gear 56 tend to be damaged. Thus, a problem exists that damage occurs to the teeth caused by interference of the teeth tips 56b with a surface 69 of a supporting table due to vibrations during transfer of the gears 56 in a laid down state just after their compaction during manufacturing steps.